Radio Frequency (RF) power amplifiers are used in mobile communication devices such as cellular telephones, smart phones, personal digital assistants (PDAs), etc. for amplifying and transmitting a RF signal from the mobile communication device. In addition to the basic function of receiving and making calls from and to other mobile communication devices, the communication devices also supports a variety of services such as messaging, downloading data, images and videos, internet browsing, audio and video playback, email access, etc. To provide such services, the mobile communication device requires operation over a wide range of frequencies.
Further, in order to increase the communication bandwidth and to provide efficient communication, different communication standards have evolved. Current communication standards include Global System for Mobile communications (GSM), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), Enhanced Data rates for GSM Evolution (EDGE), etc. Each communication standard has a defined signal specification, modulation type, transmission power requirements, operating frequency bands, etc. Moreover, the operating frequency range of the communication standards varies from one geography to another across the globe. In order to provide global roaming facility for users of the mobile communication devices, there is a need for providing mobile communication devices which enable various communication standards to co-exist and provide interoperability between multiple frequency bands. A mobile communication device which can operate at multiple frequency bands and across multiple communication standards is generally referred to as a Multi-Mode Multi-Band (MMMB) communication device.
The existing technologies that enable multi-mode multi-band operation of a mobile communication device includes a Multi-Mode Multi-Band (MMMB) RF power amplifier circuit in the mobile communication device. The existing MMMB RF power amplifier circuits include a separate low voltage, low impedance RF amplifier for each operating frequency range. For example, for a dual frequency band operation, the existing MMMB RF power amplifier circuits require two low voltage, low impedance power amplifiers. One exemplary MMMB RF power amplifier circuit for amplifying five frequency bands is shown in FIG. 1. Hereinafter, an operating frequency band is interchangeably referred to as RF input signal.
FIG. 1 illustrates a conventional MMMB RF power amplifier circuit 100. Power amplifier circuit 100 comprises power amplifier modules 102a, 102b and 102c, an antenna switch 104, an antenna 106 and third harmonic filters 116a, 116b and 116c. Power amplifier module 102a comprises a power amplifier 108a, power amplifier module 102b comprises a power amplifier 108b and a power amplifier 108c and power amplifier module 102c comprises a power amplifier 108d and a power amplifier 108e. Antenna switch 104 includes first harmonic filters 110a, 110b, 110c and 110d, second harmonic filters 112a and 112b and a switch 114.
RF input signals RFIN_a, RFIN_b, RFIN_c, RFIN_d and RFIN_e are applied to an input terminal of power amplifiers 108a, 108b, 108c, 108d and 108e respectively. An output terminal of power amplifiers 108a, 108b and 108c are respectively connected to an input terminal of each of third harmonic filters 116a, 116b and 116c. Further, a coupler is connected to an output terminal of power amplifier modules 102a and 102b. Furthermore, an output terminal of power amplifiers 108d and 108e are respectively connected to an input terminal of each of second harmonic filters 112a and 112b. An output terminal of each of third harmonic filters 116a, 116b and 116c and an output terminal of each of second harmonic filters 112a and 112b are respectively connected to separate input ports of switch 114 of antenna switch 104. An output port of switch 114 of antenna switch 104 is connected to antenna 106. Moreover, an output terminal of first harmonic filters 110a, 110b, 110c and 110d are respectively connected to separate input ports of switch 114 of antenna switch 104. An input terminal of first harmonic filters 110a, 110b, 110c and 110d are respectively connected to receiver ports RX_d, RX_e, RX_f and RX_g. Also, a first terminal of third harmonic filters 116a, 116b and 116c are respectively connected to receiver ports RX_a, RX_b and RX_c.
Power amplifier circuit 100 operates both in transmitter mode and a receiver mode. For operation as the transmitter, power amplifier circuit 100 receives RF input signals and transmits the amplified RF input signals to antenna 106. For operating as a receiver, power amplifier circuit 100 receives RF signals from antenna 106 and transmits RF signals to receiver ports RX_a, RX_b, RX_c, RX_d, RX_e, RX_f and RX_g. Power amplifier 108a amplifies the RF input signal RFIN_a, power amplifier 108b amplifies the RF input signals RFIN_b, power amplifier 108c amplifies the RF input signals RFIN_c, power amplifier 108d amplifies the RF input signals RFIN_d and power amplifier 108e amplifies the RF input signals RFIN_e respectively. Hence, power amplifier circuit 100 receives five frequency bands and includes five power amplifiers for amplifying each frequency band when operating as a transmitter.
Power amplifier circuit 100 further includes antenna switch 104 for selectively connecting RF signals to and from antenna 106. Antenna switch 104 includes first harmonic filters 110a, 110b, 110c and 110d which filter a corresponding RF signal received from antenna 106 through switch 114. The filtered RF signals are respectively transmitted from first harmonic filters 110a, 110b, 110c and 110d to the corresponding receiver ports RX_d, RX_e, RX_f and RX_g when operating as a receiver. Antenna switch 104 further includes second harmonic filters 112a and 112b which filter RF input signal RFIN_d and RFIN_e and transmit them to antenna 106 through switch 114 when operating as transmitter.
Power amplifier circuit 100 further includes third harmonic filters 116a, 116b and 116c which filter RF input signal RFIN_a, RFIN_b and RFIN_c and transmit them to antenna 106 through switch 114 when operating as transmitter. Further, third harmonic filters 116a, 116b and 116c filter a corresponding RF signal received from antenna 106 through switch 114 and respectively transmit the filtered RF signals to the corresponding receiver ports RX_a, RX_b, and RX_c when operating as a receiver.
Although FIG. 1 illustrates a MMMB power amplifier circuit 100 having five power amplifiers for amplifying five RF input signals, however the number of power amplifiers is not limited to five. The number of power amplifiers used in a MMMB power amplifier circuit increases with increase in the number of operating frequency bands required. As a result, the complexity of design and size of the MMMB power amplifier circuit and hence size of mobile communication devices in which the MMMB power amplifier circuit is used also increases.
Moreover, the existing MMMB power amplifier circuits include power amplifiers which have low output impedance and operate off a low voltage power supply such as a battery of the mobile communication device. Amplification efficiency of the power amplifier is maximized to extend the duration for which the power supply such as the battery of the mobile communication device can operate, hereinafter referred to as battery life. The efficiency of a power amplifier depends on the output power delivered by the power amplifier to a load. The output power of a power amplifier (Pout) is a function of a bias voltage (Vcc) applied to the power amplifier and the output impedance (Rs) of the power amplifier. The output power of the power amplifier can be expressed by equation 1 as:Pout=Vcc2/(2RS)For example, when the output power (Pout) required from a power amplifier is 4 W and the bias voltage (Vcc) provided by the power supply is 3.6V, the output impedance (RS) of the power amplifier is 1.62 ohm according to equation 1.
To maximize the output power (Pout) delivered by the power amplifier, the output impedance (RS) of the power amplifier should be equal to a load impedance (RL) of the load. The value of the load impedance depends on a return loss at an output terminal of the power amplifier. For the existing RF communication technologies, the load impedance is generally 50 ohm when the return loss at the output of the power amplifier is less than −10 decibels, i.e. Θ10 db.
As described above, to provide 4 W output power the output impedance (RS) of the power amplifier is 1.62 ohm which is unequal to the 50 ohm load impedance. As a result, impedance matching is required to make the output impedance (RS) of the power amplifier equal to the load impedance (RL) of the load. Usually, an impedance matching circuit comprising of one or more series inductors, one or more shunt capacitors or a combination thereof is used for transforming the load impedance (RL) of the load to match the output impedance (RS) of the power amplifier. The series inductors used in the impedance matching circuits consume power and reduce the amplification efficiency of the power amplifier. For matching high load impedance to a low output impedance of power amplifier, a large value of additional impedance will be required. Adding a series inductor with large impedance will result in more power loss and hence reduced efficiency of the power amplifier circuit. As a result, the battery life will be reduced and the power source will require frequent charging.
Moreover, the impedance matching circuits operates over a narrow frequency band. For example, a combination of series inductors and shunt capacitors selected for a low frequency band do not allow operation at high frequency bands. As a result, the power amplifier will not be able to operate as a multimode multiband power amplifier. Further, the series inductor occupies a large space when fabricated on a semiconductor die which increases the overall size of the power amplifier circuit. As a result, the size of the mobile communication device also increases. However, the existing technologies for making the mobile communication devices focus on reducing the size of the mobile communication device.
In view of the above, there is a need for a compact multimode multiband power amplifier having simple design and fabrication. Further, a need exists for a multimode multiband power amplifier having high impedance which eliminates the need of impedance matching for maximizing the output power of the power amplifier.